Set GObservation::npred_spec integration precision to 1e-8.

An offset has been observed in the fitted normalization of file function models that has been tracked down to an insufficient precision in the Npred integrator (see #1142). Numerical tests have shown that increasing the precision from 1e-5 to 1e-8 solve the issue.
Remove const version of GCTAInstDir::dir() method.

Python cannot handle const declarations, hence this method was obsolete.
Set GObservation::npred_spec integration precision to 1e-6 and adapt unit tests.

A precision of 1e-8 leads to an inacceptable increase of the computing time. A precision of 1e-6 should also to the job, although results are less accurate.
Use std::log() instead of log()
Merge branch '00-08-02' into integration